1. Field Of The Invention
This invention relates generally to free piston machines, such as free piston Stirling engines, free piston heat pumps, including coolers and cryocoolers, and linear compressors and pumps, and particularly relates to free piston machines which are connected to an AC linear electric motor or alternator. The invention is an apparatus for centering the free piston and its associated structures before an oscillating dynamic drive is applied to the free piston machine.
2. Description Of The Related Art
The prior art has and will continue to provide a broad spectrum of free piston machines. In a free piston machine, a piston linearly oscillates in a cylinder. However, the piston is free in the sense that the opposite ends of its displacement are unrestrained by mechanical linkages, such as cranks and rods or other motion transmitting structures which have components moving along specific limited paths. Ordinarily the amplitude of the reciprocation of a free piston varies as a function of loading and power. In some free piston machines the free piston is a part of the load to which power is directed and in others it is a part of the prime mover or drive. Usually the free piston is mechanically linked to other structures which reciprocate with the piston. Such other structures include linear electric motors or linear alternators. Therefore, the term "free piston member" is used herein to designate the combination of the free piston and the other component structures which are mechanically linked to the piston for reciprocating with it.
Because the entire piston member is free, there are conditions under which it will collide with structures which are positioned beyond either or both ends of its intended range of reciprocating motion. Such structures are herein termed "end walls". The term "end wall" refers generically to structures which may be struck if the piston moves beyond the intended limits of its axially oscillating reciprocation.
The need to avoid the collision of a piston member with an end wall is particularly acute in machines in which the piston is subjected to time varying gas pressures in two fluid spaces, each space bounded by an opposite end of the piston. When a free piston linearly oscillates in its reciprocating motion, it does so in opposite directions from an average position centered between its peak, axially opposite excursions. Because some net gas leakage through the gap between the piston and cylinder walls usually occurs from one fluid space to the other during each cycle of reciprocation, the gas masses in the two spaces gradually change. This change causes the center of reciprocation to creep toward the space with decreased mass. If this creep continues, the piston center position will eventually be displaced sufficiently far that the piston will collide with an end wall at one peak of its oscillating motion.
In order to avoid this consequence during the dynamic, steady state operation of a free piston machine, a variety of dynamic centering structures are used. These centering structures include both structures for maintaining a fixed center of reciprocation, as well as structures for limiting the end movement of the piston member to maintain piston member displacement within a safe range. Thus, the term "centering", as used herein, refers both to structures for maintaining a specific, fixed, center of reciprocation, as well as confining piston member displacement within range limits spaced sufficiently from the end walls to prevent collision. A few examples of dynamic centering structures are shown in the following patents, which are incorporated herein by reference: U.S. Pat. Nos. 4,183,214; 4,404,802; 4,583,364; 5,461,859; 5,385,021, 5,537,820; and 5,873,246.
Most free piston machines are operated at or near resonance because efficiency is much greater. Mechanical resonance requires a mass drivingly linked to an energy storage device, such as a spring. Therefore, the piston member is designed to have a mass which is mechanically resonant at the operating frequency of the free piston machine. The piston is linked to the housing by one or more springs. Such springs include helical and/or planar springs, gas springs, magnetic springs and the spring effect of compressible fluids in the spaces bounding the ends of the piston.
There are a variety of free piston machines. These include Beale-type free piston Stirling engines and heat pumping apparatus, such as coolers and cryocoolers, as well as free piston, linear compressors and pumps. Often these free piston machines have their piston mechanically linked to an AC electromagnetic linear transducer, such as a linear motor or linear alternator or other electrical generator. For example, a free piston Stirling engine linked to an alternator is able to use the heat of the external combustion of fuels to generate electricity and a free piston heat pumping apparatus linked to a linear motor can be used for cooling and cryocooling. An AC linear motor can drive a free piston in a cylinder having inlet and outlet valves to provide a gas compressor or fluid pump.
An AC electromagnetic, linear transducer, whether a motor or alternator, generally consists of a coil mounted to a housing for the machine and either a second coil electrically connected to operate as an electromagnet, or preferably a permanent magnet, mounted to the piston as a component of the piston member. There are also practical linear transducers of the moving coil type in which the coil is mounted to the piston as a component of the reciprocating piston member and the magnet, either a permanent magnet or an electromagnet, is mounted to the housing. The term "magnet" is used herein to refer collectively to both a permanent magnet and an electromagnet. Although an AC linear motor can also be formed by a coil and a high permeability, ferromagnetic material to form a reluctance motor operating like a solenoid, these are inefficient and therefore not preferred.
Examples of devices using linear alternators or linear motors are shown in the following U.S. Patents, which are hereby incorporated by reference: U.S. Pat. Nos. 4,602,174; 4,642,547; 4,649,283; 5,642,088; 4,912,409; 4,926,123; and 5,148,066.
When a free piston machine begins operation, it makes a transition from a static to a dynamic condition by initially linearly oscillating at small amplitudes of oscillation. The amplitude of these oscillations continues to increase until the machine reaches its steady-state operating conditions. Since most free piston machines are provided with mechanical or other springs which hold the free piston member away from the end walls under static conditions, these small transitional oscillations do not cause end wall collisions. As the oscillations grow, the dynamic centering structures, described above, for preventing end wall collisions during steady-state operation come into effect and prevent end wall collisions.
However, some free piston machines do not have springs which hold the static piston member away from end walls when the machine is not operating. Consequently, they require something to maintain the linearly oscillating piston member sufficiently far from the end walls during start-up to avoid collision. This is a particular problem for free piston machines in which the axis of reciprocation is vertical or has a vertical component of orientation. In such machines, gravitational forces, sometimes assisted by vibrations, cause the piston member to slide against an end wall when the machine is not in operation. If startup is initiated when the piston member is at or near an end wall when an oscillating dynamic drive is applied to the free piston machine, the piston member will, of necessity, repeatedly bang or collide against the end wall until the amplitude of oscillation is sufficiently large that the dynamic centering structure becomes operable. Because many of these dynamic centering structures rely upon one or more ports in the piston and cylinder walls coming into registration when the piston is in a centered position, a substantial time interval can pass before the piston oscillation amplitude becomes sufficiently large that centering can be effected.
There is, therefore, a need for a static centering apparatus for use with those free piston machines in which the piston member is capable of creeping toward an end wall when the machine is not operating so that the static centering apparatus will prevent periodic collisions during start-up of the free piston machine.
It is therefore an object and feature of the present invention to provide an apparatus to move a static piston member from its rest position into a position at which the dynamic centering system will become operable during start-up in order to avoid end wall collisions.